This invention relates to cleaning of the surface of a body of liquid. More particularly, the invention relates to a method and apparatus for recovering floating pollutants, such as oil spills, from the surface of open or confined waters.
The invention is particularly useful for recovering oil floating on water and will therefore be described with particular reference to its application for that purpose. However, the invention is not limited to recovering oil or other liquid pollutants, and the term pollutant as used herein should be understood to include, in addition to liquid substances of high or low viscosity, more or less solid debris or objects of relatively small dimensions.
WO97/07292 discloses a method and apparatus for skimming a surface portion of a body of water to flow into an immersed collection vessel for the purpose of cleaning the water surface. A pollutant, especially oil, floating on the water surface surrounding the collection vessel is caused to flow into a water sink formed in the collection vessel and allowed to separate from the water by virtue of its buoyancy and build up a layer on the water surface of a separation zone. The layer so formed on the surface of the water sink is intermittently or continuously withdrawn from the separation zone and disposed of in any suitable manner.
A surface portion of the surrounding water carrying the pollutant is caused to flow across the crest of a vertically self-positioning skimming weir which is preferably annular. To this end, water is withdrawn from the lower region of the collection vessel to maintain the water sink in the collection vessel so that the level of liquid in the collection vessel is below the level of the surrounding water surface.
In operation, the skimming weir, which possesses some buoyancy, takes an overflow position relative to the surrounding water surface such that the inflow into the collection vessel is automatically regulated to match the outflow of water from it.
The separation zone includes a generally round area of the water surface in the water sink and the water below that area. At the top portion thereof, the separation zone is horizontally spaced inwardly from the weir and preferably separated from the weir by an annular baffle serving to prevent the pollutants from entering the separation zone without first being deflected downwardly.
A characteristic feature of the known method and apparatus is that the separation of the pollutants carried by the water into the collection vessel takes place in a step which is subsequent to the skimming. In other words, there is no provision for ensuring that the liquid entering the collection vessel includes substantially only the pollutants. Instead, large amounts of water are allowed to enter the collection vessel and by ensuring that the subsequent separation step efficiently separates the pollutants from the mixture of water and pollutants a concentrated pollutant layer is obtained in the separation zone.
If the pollutants have a density which is only slightly lower than that of the water, as is the case with most pollutants consisting of petroleum products, an efficient separation of the pollutants from the mixture of water and pollutants is possible only if the flow velocity of the mixture is very low.
It has been found that the mixture of water and pollutants flowing across the skimming weir like a waterfall or cascade tends to flow more or less vertically downwardly as a relatively confined stream in the liquid contained in the collection vessel. Moreover, it has been found that the stream loses its momentum only slowly, so that it can easily reach the bottom region of the collection vessel, where water is discharged.
It is important, therefore, to provide for the flowing mixture to reduce its velocity drastically so that the separation in the separation zone can take place efficiently. In the above-described known apparatus a reduction of the velocity can be brought about by widening the separation zone and allowing the downwardly flowing mixture to deflect at least partially into the separation zone and distribute there across a large area.
However, in the known method and apparatus, widening of the separation zone also means that the annular skimming weir has to be widened. This in turn means that the inflow across the skimming weir is increased and that the reduction of the velocity in the separation zone will therefore be limited.
This limitation is avoided in the method and apparatus according to the invention the features of which are set forth in the independent claims and preferred embodiments of which include features set forth in the dependent claims.
As will become apparent as the description proceeds, in the method and apparatus according to the invention the separation zone is horizontally offset from the region beneath the water sink in the collection vessel. This is in contrast to the prior art as described above, in which the separation zone is within the region beneath the water sink in the collection vessel.
In one preferred embodiment of the invention the separation zone is annular and surrounds the region beneath the water sink. It can then be expanded horizontally virtually without limitation so that its horizontal cross-sectional area can readily be chosen as desired to produce the desired low flow velocity. Expansion can be accomplished without any expansion the skimming weir and the water sink being necessary. The downward flow of the mixture of water and pollutant can be deflected radially outwardly in all directions and spread across a horizontal cross-sectional area the size of which is a function of the square of the distance of the outer perimeter from the centre.
In accordance with a second preferred embodiment, the separation zone is provided in a separation vessel which is horizontally spaced apart from the collection vessel and connected to it through a conduit through which the mixture of water and pollutant is conveyed from the collection vessel to the separation vessel in which the accumulation of the pollutants on a water surface takes place. Both the collection vessel and the separation vessel may be positioned in the body of water which carries the pollutant.
Preferably, the stream of water carrying the pollutant is introduced into the separation vessel through a vertically extending, upwardly opening outlet conduit and, likewise preferably, the upwardly directed stream exiting from the outlet conduit is diverted downwardly through an annular zone which surrounds the outlet conduit and the lower end of which communicates with the separation zone.
In this second embodiment as well, the horizontal dimensions of the separation zone can readily be chosen virtually as desired to present a cross-sectional area to the stream entering the separation zone which is large enough to reduce the flow velocity very substantially.
The invention will be described in greater detail below with reference to the accompanying diagrammatic drawings and exemplary embodiments of the method and apparatus according to the invention illustrated therein.